In certain machining processes, it is desirable to very slowly (a few tens of microinches per minute at most) and at a controllable rate remove material from a flat or relatively flat workpiece work surface. One application where this capability is particularly useful is in machining a new type of magnetic transducer head employed by data recording devices, such as disk memory drives. These are known as thin-film transducers or heads.
In the past, the magnetic transducers employed by disk memory drives for writing data onto the individual disks and reading the data back therefrom have been formed with ferrite cores having small windings placed around one leg. The difficulty in producing such cores placed an effective limit on the core's size and flux gap width and length, which places limits on the width and linear bit density of the data track written. The shorter the flux path and the narrower and shorter the flux gap, the more densely can data be recorded.
Recent approaches to the fabrication of such transducers have used thin film technology to create the tranducers. Such transducers are formed of individual layers of insulating material, conductive material, and magnetic flux-conducting material created by successive deposition steps. The position and shape of features being formed of each particular material deposited is controlled by masks. Such deposition technology is old in the art, having been used in the fabrication of electronic circuitry for many years. In essence, the circuit fabrication technology employing deposition is used to create a magnetic core and a winding of the appropriate characteristics on the side of an aerodynamic slider, allowing the transducing of the data signals onto and from the disk in a disk memory. While the relative positioning and size of individual features of a single pattern being created is highly accurate, the accuracy of registration between succesive patterns employed in forming the layers comprising a complete transducer is less accurate. And the accuracy of registration of the patterns relative to a datum line on a substrate is even less accurately controllable.
When dealing with thin film heads it is necessary to control the throat height of the flux gap very accurately in order to control its magnetic characteristics. (Throat height is the dimension of the flux gap normal to the aerodynamic or flying surface of the head, and the parallel recording surface.) It is now desirable to control throat height to an accuracy of 60 microinches (1.5 microns) or less.
The use of thin film deposition techniques is substantially less costly when many patterns or elements are formed simultaneously. Therefore, it is usual to create perhaps hundreds of thin film heads simultaneously on a wafer substrate. The substrate is then sliced to create bars each having on a side a number of heads with their flux gaps aligned along one edge. This edge is formed by the intersection of the side carrying the heads with the surface aligned with the flux gaps. The surface aligned with the flux gaps forms the flying surfaces of the heads which float on a thin air bearing above the disks. (Typically, in a final step of the process, the bars will be diced into individual sliders, each having one or two transducers.)
The slicing of the substrate into these individual bars cannot be controlled with any great accuracy. Cutting these bars from the substrate cause stress changes in the bars which shift the relative positions of the transducers. Accordingly, after the individual bars have been cut from the wafer there will be small but significant on a microinch scale, variations in the throat heights of the transducers carried along the bar. Furthermore, as already mentioned, the deposition techniques have not always arranged the positions of the flux gaps of the adjacent transducers on the bar with precision relative to each other or to any datum line. Lastly, the simple process of mounting the individual bar on a carrier for machining, for example by adhesive bonding, creates stress causing additional variations in spacing from a datum.
Accordingly, it is necessary to machine the flying surfaces until the flux gap throat heights of each individual head are within the desired tolerance. To accurately control the machining of these individual sliders, it is usual to set these throat heights by machining each bar sliced from the original wafer. To aid in determining the throat heights of the individual flux gaps, so called machining sensors have been in use in conjunction with a conventional workpiece support which holds the workpiece against a lapping wheel. Output from the machining sensors is monitored until the outputs indicate that the heads, or at least the maximum possible of them, have achieved the proper throat height.
In the prior art, this machining is done in some cases by use of a workpiece holder which mechanically advances the workpiece towards the grinding surface along a preselected path. The workpiece position along that path can be controlled to permit the desired amount of material to be abraded from the workpiece. U.S. Pat. Nos. 3,110,136 (Spira), 3,921,340 (Johnson et al.), 4,014,141 (Riddle et al.), and 4,062,659 (Feierabend et al.) teach machining techniques of this type. Runout in the abrasive-carrying surface and the workpiece holder employed by these approaches makes achieving the accuracy in the 60 microinch (1.5 microns) tolerance range difficult.
In other cases, a free carrier floats on the lapping surface supported by wheels or lands and held in place by a fixed restraint. These devices do not easily allow connection of on-board machining sensors to external electronics, and the constant abrasion on the carrier requires frequent replacement of the support elements.